1. Field of the Invention
The present invention relates to an image sensor and an imaging apparatus.
2. Description of the Related Art
Image sensors such as CCD image sensors and CMOS image sensors include a one- or two-dimensional pixel array with a plurality of pixels. In recent years, the number of pixels in the pixel array of an image sensor has increased in order to capture high-definition images. Some image sensors for use in imaging apparatuses, such as digital video cameras or digital still cameras, now have as many as several million pixels or more in their pixel arrays.
Each pixel in a pixel array generates an image signal when incident light reaches a light receiving surface of a photodiode and the photodiode photoelectrically converts the received light. Thus, it is important to efficiently guide the light incident on a pixel to a photodiode in the pixel. In other words, assuming that “light utilization efficiency” is a ratio of an amount of light reaching a photodiode to an amount of light incident on a pixel, an improvement of the light utilization efficiency has a significant effect on improvements in image sensor performance.
Also, with increasing needs to develop more compact imaging apparatuses and smaller image sensors in recent years, image sensors are required to ensure sufficient light utilization efficiency not only with light incident at a small angle but also with light incident at a large angle.
To meet this demand, Japanese Patent Laid-Open No. 2002-246579 has described a configuration of a solid-state imaging device in which a non-through hole 108 formed by removing a portion above the photoelectric conversion element 100 from an interlayer insulator film 102 is filled with a transparent material 203 that has a refractive index higher than that of the interlayer insulating film 102, as illustrated in FIG. 3 of Japanese Patent Laid-Open No. 2002-246579.
In addition, Japanese Patent Laid-Open No. 2002-118245 has described a configuration of a solid-state imaging device in which an optical waveguide hole 5 formed by removing a portion above a surface of a light receiving unit 2 from an insulating layer 3 is filled with a material 13 that has a refractive index higher than that of the insulating layer 3, as illustrated in FIG. 3 of Japanese Patent Laid-Open No. 2002-118245.
In the configuration illustrated in FIG. 3 of Japanese Patent Laid-Open No. 2002-246579, the side face of the waveguide is made of one kind of material.
In the configuration illustrated in FIG. 3 of Japanese Patent Laid-Open No. 2002-118245, only the insulating layer 3 is in contact with the material 13 at the side wall of the optical waveguide 5a. 
With this configuration, if the insulating layer 3 is formed of a material that has a refractive index sufficiently lower than that of the material 13, it may be difficult to ensure a required mechanical strength of the insulating layer 3 when a material with a sufficiently low refractive index has low mechanical strength.
If the insulating layer 3 is formed of a porous CDO (carbon-doped oxide) that has a refractive index of 1.3, a difference in refractive index between the material 13 and the insulating layer 3 at the side face of the optical waveguide 5a is larger than in the case where the insulating layer 3 is formed of a silicon oxide that has a refractive index of 1.4. However, in this case, it may be difficult to ensure the required mechanical strength of the insulating layer 3.